1. Field of the Invention
The present invention relates to a transversal surface acoustic wave filter, and particularly, to a transversal surface acoustic wave filter including a shield electrode disposed between an input-side IDT electrode and an output-side IDT electrode.
2. Description of the Related Art
Transversal surface acoustic wave filters have been widely used as band-pass filters for video intermediate frequencies (VIF) of television receivers. In such a surface acoustic wave filter, a shield electrode is frequently disposed between an input-side IDT electrode and an output-side IDT electrode and connected to a ground potential. The shield electrode suppresses electromagnetic and electrostatic coupling between the input-side IDT electrode and the output-side IDT electrode.
In recent years, there has been strong demand for miniaturization of surface acoustic wave filters. In order to advance the miniaturization of a transversal surface acoustic wave filter, the distance between an input-side IDT electrode and an output-side IDT electrode must be decreased. However, a decrease in the distance causes a problem of producing an electric field between the shield electrode and the input-side IDT electrode, which causes an unwanted wave. In other words, the unwanted wave is coupled with a surface acoustic wave to be used on the output-side IDT electrode side, thereby causing a problem of producing a large ripple in the pass band.
Therefore, Patent Document 1 discloses a surface acoustic wave filter having a structure for suppressing the unwanted wave. FIG. 5 is a schematic plan view illustrating a surface acoustic wave filter disclosed in Japanese Examined Patent Application Publication No. 2-44168 (Patent Document 1).
A surface acoustic wave filter 101 includes a shield electrode 104 disposed between an input-side IDT electrode 102 and an output-side IDT electrode 103. Furthermore, two floating electrode fingers 105a and 105b with a line width of about λ/8 are disposed, with a space of about λ/8 therebetween, between the shield electrode 104 and the input-side IDT electrode 102. In addition, λ represents the wavelength of a surface wave used in the surface acoustic wave filter 101.
The excitation point of the unwanted wave is at an intermediate point between the input-side IDT electrode-side of the shield electrode 104 and the shield electrode-side end of the input-side IDT electrode 102. Therefore, when the floating electrode fingers 105a and 105b are arranged to sandwich the intermediate point therebetween, an excitation electric field of the unwanted wave is canceled.
In the surface acoustic wave filter 101 disclosed in Patent Document 1, a split-type floating electrode including the floating electrode fingers 105a and 105b is provided to cancel an excitation electric field of an unwanted wave, thereby suppressing a ripple caused by the unwanted wave. However, in the surface acoustic wave filter 101 disclosed in Patent Document 1, the unwanted wave produced when the input-side IDT electrode 102 is brought close to the shield electrode 104 can be suppressed, but a re-excited wave is produced by providing the split-type floating electrode, thereby causing a problem of producing a ripple due to the re-excited wave. More specifically, as described in Patent Document 1, when a split-type floating electrode including the floating electrode fingers 105a and 105b is provided, an undesired ripple inevitably occurs in the pass band.